1. Field of the Invention
This invention relates to stabilized lithium manganese oxide spinel battery cathode material, and to improved methods of stabilizing the spinel against acid attack and the like.
2. Description of the Prior Art
Recently, there has been increased interest in using lithium manganese oxide having the formula Li1+xMn2xe2x88x92xO4 (0.02xe2x89xa6xxe2x89xa60.15, unless stated otherwise), referred to in the art as spinel or LMO, as a cathode material in lithium-ion batteries. The advantages of using spinel instead of the more commonly used alternatives, i.e., LiCoO2 or Li(Co, Ni)O2, are well known. For example, spinel is less expensive, environmentally friendly and considerably safer during operation than the alternative materials. However, the use of spinel as battery cathode material has major drawbacks, i.e., the spinel exhibits a rapid loss in capacity when cycled or stored at temperatures above 45xc2x0 C. and mineral acid impurities in batteries degrade the spinel and reduce its performance.
A variety of solutions to the problem of the rapid loss in capacity of spinel above 45xc2x0 C. have been proposed by those skilled in the art. The solutions include the incorporation of additional lithium into the spinel lattice to form spinel of the formula Li1+xMn2xe2x88x92xO4 or substituting fluoride for some of the oxygen to yield spinel having the formula Li1+xMn2xe2x88x92xO4xe2x88x92zFz (see Amatucci et al., U.S. Pat. No. 5,674,645 and Sugiyama et al., U.S. Pat. No. 6,087,042). Another solution involves replacing a fraction of the Mn with a stabilizing metal (M) such as Cr, Ni, Co, Al and the like to form Li1+xMyMn2xe2x88x92xxe2x88x92yO4 (Dahn et al., U.S. Pat. No. 5,900,385) wherein x is greater than 0 but less than 1 and y is less than or equal to 1.
Another proposed solution involves the formation of a protective coating on the particles of spinel to prevent corrosion or dissolution of the spinel. The formation of a protective coating on spinel is disclosed in U.S. Pat. No. 5,443,929 issued to Yamamoto et al. on Aug. 22, 1995 wherein a lithium-deficient spinel (Li1+xMn2O4) encapsulated by lithium manganite (Li2MnO3) is disclosed. In accordance with the teachings of the patent, LiOH powder is added to stoichiometric spinel at various ratios from 0.02:1 to 1.2:1 and the mixture is heated in air at 200xc2x0 C. to 1000xc2x0 C., preferably at 375xc2x0 C., for 20 hours. The final product is a dual phase material that has acid resistance and enhanced stability during high temperature battery operation, but it suffers from the disadvantage that the maximum capacity is substantially reduced.
U.S. Pat. No. 5,733,685 issued to Wang on Mar. 31, 1998 and U.S. Pat. No. 5,783,328 also issued to Wang on Jul. 21, 1998 disclose that improved spinel cathode material stability is obtained by protecting the spinel particles with a thin layer of lithium carbonate (Li2CO3). The coating is accomplished by combining a solution of LiOH with spinel having the formula Li1+xMn2xe2x88x92xO4 wherein x is greater or equal to 0 and less than or equal to 0.1. After the mixture is dried, it is heated to a temperature of from 270xc2x0 C. to 300xc2x0 C. for 20 hours in the presence of carbon dioxide. While the resulting layer of Li2CO3 on the spinel makes it more robust than unprotected spinels at temperatures above 45xc2x0 C., the coated spinel tends to out-gas during battery use which causes the battery case to swell or vent, etc.
U.S. Pat. No. 5,705,291 issued to Amatucci et al. on Jan. 6, 1998 discloses that a glassy coating of LiOH mixed with B2O3 and other additives retards spinel capacity loss, and U.S. Pat. No. 6,022,641 issued to Endo et al. on Feb. 8, 2000 discloses the benefits of mixing Li2CO3 or Na2CO3 with spinel in an amount of from 0.5% to 20% by weight of the spinel to improve cycle performance. Further, Oesten et al. (WO 00/70694xe2x80x94Nov. 23, 2000) protect all lithium metal oxide cathode materials by coating the active particles with an organometallic species and then pyrolyzing to leave a metal oxide outer layer.
It is also possible to coat lithium manganese oxide spinels with other battery-active cathode materials having the general formula LiMOx wherein M is a transition metal (Iguchi et al., Japanese Kokai HEI 8[1996]-162114 and Hwang et al., U.S. Pat. No. 5,928,622). In this approach, thermally decomposable Li and M salts (or oxides) in the appropriate Li:M ratio are blended with the spinel and reacted at temperatures up to 750xc2x0 C. This results in particles of the original spinel having an acid resistant LiMOx-rich outer shell.
The surface treatments of spinel battery cathode material of the types described above inevitably result in a decrease of the maximum reversible discharge capacity of the spinel. In addition to lowered capacity caused by the added mass of electrochemically inert species, Gummow et al. in Solid State Ionics, 69, 59 (1994) showed that the inclusion of non-stoichiometric Li in Li1+xMn2xe2x88x92xO4 will decrease the discharge capacity of the resulting material by a factor of (1-3x). The advantage of such treatments is that they slow the loss of discharge capacity (referred to in the art as fade) during repeated charge/discharge cycles. The battery industry""s yardstick of useful battery life dictates battery replacement when the battery has lost 20% of its initial discharge capacity. The protective coatings extend the number of useful cycles that the spinel cathode material can provide, but as mentioned, the maximum reversible discharge capacity of the spinel is significantly decreased.
The prior art discloses providing spinel battery cathode material with a coating of an acid resistant or acid-scavenging compound. Mineral acids present as impurities in lithium ion batteries attack Li1+xMn2xe2x88x92xO4 spinel cathode material extracting lithium and up to 25% of the manganese leaving the spinel unable to perform satisfactorily. The impurity acids are generated in lithium-ion batteries by the hydrolysis of LiPF6 electrolyte salt, by trace moisture or by the oxidation of organic carbonate electrolyte solvents at the high voltage end of the battery cycle. While the protection afforded by the above described prior art coatings prevent or reduce the problems associated with acid attack at temperatures below 45xc2x0 C. in batteries for mobile devices such as cell phones, laptop computers, photographic equipment and the like, the prior art coatings significantly reduce the maximum reversible discharge capacity of the spinel cathode material.
Thus, there is a need for an improved method of treating particles of spinel battery cathode material to produce a protective coating on the spinel particles which protects the spinel from acid attack, reduces the loss of maximum discharge capacity only a minimal amount and substantially improves the fade rate during charge/discharge cycles.
The present invention provides improved stabilized particulate Li1+xMn2xe2x88x92xO4 spinel battery cathode material and methods of treating particles of Li1+xMn2xe2x88x92xO4 spinel to produce a protective coating of a battery inactive, ceramic like lithium metal oxide on the particles. The coating resists acid attack, substantially improves the capacity fade of the material and reduces the maximum discharge capacity of the material only minimally. The methods of the invention are basically comprised of the following steps. The spinel particles are mixed with a particulate reactant selected from a lithium salt, a lithium metal oxide or a mixture of a lithium salt and a metal oxide. Thereafter, the resulting particulate mixture is heated for a time in the range of from about 15 minutes to about 20 hours at a temperature in the range of from about 350xc2x0 C. to about 850xc2x0 C. During the heating step, the particulate lithium salt, lithium metal oxide or mixture of the lithium salt and a metal oxide reacts with the spinel particles whereby a protective coating of battery-inactive lithium metal oxide is formed on the spinel particles and the lithium content of the spinel particles adjacent to the coating is increased a limited amount as represented by the formula Li1+xMn2xe2x88x92xO4 wherein x is less than 0.2.
The coated spinel particles are preferably cooled to a temperature below 200xc2x0 C. in a time period in the range of from about 10 to about 120 minutes. Thereafter, the spinel particles are cleaned and sized by removing agglomerates and metallic particles while passing the particles through a magnetic trap and 150 mesh or smaller screens.
It is, therefore, a general object of the present invention to provide improved stabilized spinel battery cathode material and methods of treating spinel particles to produce a protective coating of lithium metal oxide on the particles without substantially reducing the maximum discharge capacity of the resulting material while improving the capacity fade of the material.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.